小兴安岭构造带早白垩世最晚期镁铁质侵入岩的岩石成因及地质意义

Petrogenesis and geological significance of latest Early-Cretaceous mafic intrusion in Lesser Xing'an Range,China

丰丽镁铁质侵入岩位于小兴安岭构造带北缘,其岩石组成主要为角闪辉长岩和辉长岩。其中角闪辉长岩中含有较大的角闪石斑晶,并可见单斜辉石和斜长石包含其中。SHRIMP锆石U-Pb定年结果显示岩体侵位年代为107±1.0Ma。丰丽岩体具有基本一致的(87Sr/86Sr)i(0.7065)和εNd(t)(-0.17~0.14)组成,表明岩浆上升过程中没有受到地壳物质的混染,且同早中生代镁铁质岩浆岩相比,丰丽岩体的Sr-Nd同位素组成较为富集,可能与古亚洲洋和古太平洋多期次俯冲交代导致大离子亲石元素和轻稀土元素的富集累积有关。全岩主微量元素组成揭示岩浆来源于亏损的岩石圈地幔,其源区物质组成为尖晶石二辉橄榄岩,岩浆经历了较低程度的部分熔融(~3%),岩浆演化过程中经历了明显的橄榄石、单斜辉石、斜方辉石及钛铁矿物的分离结晶。锆石氧同位素组成δ^(18)O值变化于5.0‰~6.6‰之间,平均值为5.8‰,略高于典型地幔锆石的δ^(18)O值,暗示岩浆源区曾受到洋壳上部俯冲沉积物熔流体交代作用影响,同时角闪石矿物的结晶说明原始岩浆含水量较高,也证明源区经历过丰富的流体交代作用的影响。早白垩世最晚期(110~100Ma),东北地区岩浆作用的空间分布范围集中在最东部地区,岩性上主要为碱性岩、双峰式火山岩及少量的镁铁质岩浆岩,说明此时东北地区处于古太平洋后撤导致的陆内伸展拉张环境中,岩石圈地幔发生减压部分熔融形成了研究区镁铁质岩浆岩。

The Fengli mafic intrusion crops out in the northern margin of the Lesser Xing'an Range.It predominantly consists of hornblende-gabbro and gabbro.The hornblende gabbro contains large hornblende phenocrysts,which contain clinopyroxene and plagioclase inclusions.The Fengli mafic pluton yielded a crystallization age of 107±1.0 Ma.They are characterized with homogeneous(87Sr/86Sr)i(0.7065)andεNd(t)(-0.17~0.14),which exclude the possibility of significant crustal contamination.Moreover,compared with the Early-Mesozoic mafic intrusions,Fengli pluton has relatively enriched Sr-Nd isotopic compositions.This could be resulted from the multi-stage Paleo-Asian Ocean and Paleo-Pacific subduction which leading to the enrichment and accumulation of the large ion lithophile elements and light rare earth elements.Whole rock major and trace element signatures suggest that Fengli mafic rocks were generated by low degree partial melting of depleted lithospheric mantle under spinel-facies conditions.Fengli mafic rocks have zirconδ^(18)O values of 5.0‰to 6.6‰(average 5.8‰),slightly higher than the typical mantle zirconδ^(18)O value,indicating that the parental magmas were likely derived from mantle wedge metasomatized by sedimentary melt and fluid from the subducted Paleo-oceanic plate.In addition,the occurrence of the hornblende in the rocks also indicates a high-H_(2)O parental magma which experienced fluid metasomatism in the mantle source.In the latest Early-Cretaceous(110~100Ma),the eastward narrowing of the distribution of igneous rocks,most of which are alkaline rocks,bimodal volcanic rocks and mafic rocks,reveals an intracontinental extensional environment resulted from the rollback of the Paleo-Pacific slab.Decompression partial melting of the lithospheric mantle induced the occurrence of the Fengli mafic rocks.